Abstract
A slip ring unit for an electric generator is provided. The slip ring unit includes a slip ring attachable to a rotor shaft of the electric generator, a plurality of sliding contacts arranged along a circumference of the slip ring, to provide an electrical connection with the slip ring, at least one temperature sensor for measuring a temperature inside the slip ring unit, a fan for providing a cooling flow in the slip ring unit, and a controller connected to the fan for controlling the cooling flow rate generated by the fan, the controller being connected to the at least one temperature sensor. The at least one temperature sensor is attached to at least a holder for supporting the sliding contacts. The controller is configured in such a way that the cooling flow rate is generated depending on the temperature measured by the at least one temperature sensor.
Claims
1. A slip ring unit for an electric generator comprising: a slip ring attachable to a rotor shaft of the electric generator; a plurality of sliding contacts arranged along a circumference of the slip ring to provide an electrical connection with the slip ring; at least one temperature sensor for measuring a temperature inside the slip ring unit, at least one fan for providing a cooling flow in the slip ring unit, and a controller connected to the at least one fan for controlling a cooling flow rate generated by the at least one fan, the controller being connected to the at least one temperature sensor, wherein the at least one temperature sensor is attached to at least a holder for supporting the plurality of sliding contacts and the controller is configured in such a way that the cooling flow rate is generated depending on the temperature measured by the at least one temperature sensor.
2. The slip ring unit according to claim 1, wherein the controller comprises an input for receiving a temperature signal from the at least one temperature sensor and an output for sending a controlling signal to the at least one fan, the controlling signal being a function of the temperature signal.
3. The slip ring unit according to claim 2, wherein the controlling signal is proportional to the temperature signal.
4. The slip ring unit according to claim 3, wherein the controlling signal is linearly proportional to the temperature signal.
5. The slip ring unit according to claim 2, wherein the controlling signal is a voltage signal.
6. An electric generator comprising a rotor, a stator, a rotor shaft and a slip ring unit according to claim 1.
7. A wind turbine comprising an electric generator according to claim 6.
8. An operating method for operating a slip ring unit according to claim 1, the method comprising: measuring a temperature inside the slip ring unit with the at least one temperature sensor, providing a cooling flow in the slip ring unit, and controlling the cooling flow rate depending on the temperature measured by the at least one temperature sensor.
9. The operating method according to claim 8, wherein the cooling flow rate is proportional to the temperature signal.
10. The operating method according to claim 9, wherein the cooling flow rate is linearly proportional to the temperature signal.
11. The operating method according to claim 8, wherein the method further comprising: measuring a rotor speed and a rotor current; calculating thermal losses in the slip ring unit; calculating a required air flow in the slip ring unit depending on the thermal losses; calculating a plurality of temperatures in a respective plurality of points of the slip ring unit, determining at least one wear-ratio curve for the sliding contacts of the slip ring unit; and determining an expected lifetime of the sliding contacts by using the at least one wear-ratio curve and the temperature measured inside the slip ring unit.
Description
BRIEF DESCRIPTION
[0017] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:
[0018] FIG. 1 shows a schematic section of an electrical generator comprising a slip ring unit according to embodiments of the present invention;
[0019] FIG. 2 shows an axonometric view of a component of the slip ring unit of FIG. 1;
[0020] FIG. 3 shows a flux diagram illustrating an operating method for the slip ring unit of FIG. 1;
[0021] FIG. 4 shows a graph illustrating operative steps of the method of FIG. 3; and
[0022] FIG. 5 shows an embodiment of the operating method of FIG. 3.
DETAILED DESCRIPTION
[0023] FIG. 1 shows a simplified scheme of an electrical generator 10. The electrical generator 10 may be mechanically connected to a shaft driven by rotor blades of a wind turbine (not shown). The generator may be indirectly connected to an electrical grid by a converter and a transformer (not shown). In such example, the electrical generator 10 may convert the mechanical power coming from the rotor of the wind turbine to electrical power to be provided to the electrical grid. According to other examples (not shown), the electrical generator 10 may be not connected to a wind turbine but to any other source of primary power, different from the wind. The electrical generator 10 may comprise a rotor 3 and a stator 2. The rotor 3 may be radially internal to the stator 2 and may be coupled to a rotatable shaft 5, in order to be rotatable with respect to the stator 2 about a rotational axis Y. According to other examples (not shown), the rotor 3 may be radially external to the stator 2. The electrical generator 10 may be a Doubly Fed Induction Generator (DFIG). The rotor 3 and the stator 2 may comprise a rotor winding 8 and a stator winding 7, respectively. The rotor winding 8 and the stator winding 7 may be electrically independent and separately connected to a transformer (not shown), for transforming the electrical power provided by both the rotor winding 8 and the stator winding 7 to an electrical voltage compatible with the electrical grid. According to other examples, the electrical generator 10 may be another type of electrical generator. Between the rotor 3 and the converter a slip ring unit 20 may be provided, which may permit, by sliding contacts, for example brushes, to establish an electrical connection between the rotor winding 8 and other stationary electrical components. The slip ring unit 20 may comprise a slip ring 11 attached to the rotor shaft 5 and a plurality of sliding contacts 12 arranged along a circumference of the slip ring 11, to provide an electrical connection with the slip ring 11. The sliding contacts 12 may be brushes. The slip ring unit 20 may comprise one or more holders for supporting the sliding contacts 12. The slip ring unit 20 may further comprise an external housing 13 for housing the slip ring 11, the sliding contacts 12 and the holders 15.
[0024] FIG. 2 shows a plurality of sliding contacts 12 in the form of brushes distributed around the slip ring 11 (not shown in FIG. 2). The brushes 12 may be supported by a plurality of holders 15 (three holders 15 are shown in FIG. 2) in the form of arcuated plates coaxial with the rotational axis Y (see FIG. 1). The holders 15 may be axially distributed and distanced along the rotational axis Y (see FIG. 1). Each of the holders 15 may support a respective plurality of brushes 12 so that they may be held in contact with an external surface of the slip ring 11. The holders 15 may comprise at least one temperature sensor 25 (three temperature sensors 25, one for each holder 15, in the example of FIG. 2) for measuring a respective temperature inside the slip ring unit 20. The temperature sensor(s) 25 may be provided at one circumferential end of the holder 15. The measurement may be associated with temperature at the contact between the brushes 12 and the slip ring 11.
[0025] FIG. 3 shows a block diagram illustrating how a cooling flow inside the slip ring unit 20 may be controlled. The temperature sensor may measure a temperature. The temperature sensor 25 may provide a temperature signal Ta as input to a controller 31, wherein the temperature signal may be proportional to the measured temperature. The controller 31 may be connected to a fan 30 which may provide a cooling flow F in the slip ring unit 20. According to other examples (not shown), the controller 31 may be connected to two or more fans that may provide a cooling flow F in the slip ring unit 20. The controller 31 may control the cooling flow rate generated by the fan 30 depending on the temperature measured by the at least one temperature sensor 25. The controller 31 may comprise an input for receiving the temperature signal Ta from the temperature sensor(s) and an output for sending a controlling signal Vout to the fan 30. In examples comprising two or more fans, the controller 31 may send a plurality of controlling signals, i.e., one per each fan, wherein such controlling signals that may be equal or different e.g., if a plurality of sensors are used.
[0026] The controlling signal Vout may be proportional to the temperature signal Ta. In an example, the controlling signal Vout may be linearly proportional to the temperature signal Ta. The controlling signal Vout may be a voltage signal to be applied to an electric motor of the fan. By varying the controlling signal Vout the speed of the motor of the fan 30 may be changed and consequently also the rate of the cooling flow F may be changed.
[0027] FIG. 4 shows a graph illustrating two different function 50, 51 illustrating a dependency between the controlling signal Vout and the temperature signal Ta. A first linear function 50 extends between a point at a first minimum temperature T2 and Vout=0 to another point where the temperature has a first maximum value T1 and Vout=Vmax. At Vmax the speed of the fan 30 and consequently the generated cooling flow rate reaches also a maximum value. A second linear function 51 extends between a point at a second minimum temperature T4 and Vout=0 to another point where the temperature has a second maximum value T3 and Vout=Vmax. When a temperature value T5 is measured by the temperature sensor(s) 25 and provided to the controller, a first control value Vout1 of the controlling signal Vout may be generated if the first linear function 50 is used. Alternatively, a second control value Vout2 of the controlling signal Vout may be generated if the second linear function 51 is used. According to examples, other functions assessing a dependency between the controlling signal Vout and the temperature signal Ta may be used. Such functions may be linear or not linear. The adjustable cooling flow rate permits to operate the brushes at the temperature where an optimal performance is obtained or at least close to such temperature. At such operational points wear ratio of the brushes may be reduced, thus allowing the extension of the maintenance frequency intervals.
[0028] FIG. 5 shows a block diagram illustrating a method for operating the slip ring unit 20, which may be used for determining an expected lifetime of the brushes 12. In block 100, the input data may be provided. Input data may comprise operational inputs 110 and system architecture dimensional data 120. The operational inputs 110 may comprise measured values of speed and current of the rotor 3. The system architecture dimensional data 120 may comprise geometrical information about the sliding contacts 12, the slip ring 11 and the holders 15. In block 200, the thermal losses 210 of the slip ring unit 20 may be calculated. The thermal losses 210 may comprise electrical and mechanical losses. Block 200, may further comprise calculating the required air flow 220 in the slip ring unit 20, depending on the thermal losses 210. In block 300, a plurality of temperatures may be calculated in a respective plurality of points of the slip ring unit 20. Based on system architecture dimensional data 120 and the thermal losses 210, the expected temperatures in different portions of the slip ring unit 20 may be calculated. In block 400, at least one wear-ratio curve 410 for the sliding contacts 12 may be determined. Block 400, may further comprise determining an expected lifetime 420 of the sliding contacts 12 by using the wear-ratio curve 410 and the temperature Ta measured inside the slip ring unit 20. The expected temperature distribution and the point on the wear-ratio curves depending on the contact temperature Ta may be used to determine the expected lifetime, which may further be used for defining and scheduling the maintenance tasks. According to the different embodiments of the present invention, the calculations in the final block 400 may be performed depending on the calculations performed in any of the previous steps 100, 200, 300. According to the different embodiments of the present invention, a method for operating the slip ring unit 20 may include only part of the steps 100, 200, 300, 400.
[0029] Although the present invention has been disclosed in the form of embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.
[0030] For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements.
